467 research outputs found
Moment operators of the Cartesian margins of the phase space observables
The theory of operator integrals is used to determine the moment operators of
the Cartesian margins of the phase space observables generated by the mixtures
of the number states. The moments of the -margin are polynomials of the
position operator and those of the -margin are polynomials of the momentum
operator.Comment: 14 page
Signal processing in local neuronal circuits based on activity-dependent noise and competition
We study the characteristics of weak signal detection by a recurrent neuronal
network with plastic synaptic coupling. It is shown that in the presence of an
asynchronous component in synaptic transmission, the network acquires
selectivity with respect to the frequency of weak periodic stimuli. For
non-periodic frequency-modulated stimuli, the response is quantified by the
mutual information between input (signal) and output (network's activity), and
is optimized by synaptic depression. Introducing correlations in signal
structure resulted in the decrease of input-output mutual information. Our
results suggest that in neural systems with plastic connectivity, information
is not merely carried passively by the signal; rather, the information content
of the signal itself might determine the mode of its processing by a local
neuronal circuit.Comment: 15 pages, 4 pages, in press for "Chaos
Semispectral measures as convolutions and their moment operators
The moment operators of a semispectral measure having the structure of the
convolution of a positive measure and a semispectral measure are studied, with
paying attention to the natural domains of these unbounded operators. The
results are then applied to conveniently determine the moment operators of the
Cartesian margins of the phase space observables.Comment: 7 page
Chaotic Phase Synchronization in Bursting-neuron Models Driven by a Weak Periodic Force
We investigate the entrainment of a neuron model exhibiting a chaotic
spiking-bursting behavior in response to a weak periodic force. This model
exhibits two types of oscillations with different characteristic time scales,
namely, long and short time scales. Several types of phase synchronization are
observed, such as 1 : 1 phase locking between a single spike and one period of
the force and 1 : l phase locking between the period of slow oscillation
underlying bursts and l periods of the force. Moreover, spiking-bursting
oscillations with chaotic firing patterns can be synchronized with the periodic
force. Such a type of phase synchronization is detected from the position of a
set of points on a unit circle, which is determined by the phase of the
periodic force at each spiking time. We show that this detection method is
effective for a system with multiple time scales. Owing to the existence of
both the short and the long time scales, two characteristic phenomena are found
around the transition point to chaotic phase synchronization. One phenomenon
shows that the average time interval between successive phase slips exhibits a
power-law scaling against the driving force strength and that the scaling
exponent has an unsmooth dependence on the changes in the driving force
strength. The other phenomenon shows that Kuramoto's order parameter before the
transition exhibits stepwise behavior as a function of the driving force
strength, contrary to the smooth transition in a model with a single time
scale
Fermi Large Area Telescope Observations of the Cosmic-Ray Induced gamma-ray Emission of the Earth's Atmosphere
We report on measurements of the cosmic-ray induced gamma-ray emission of
Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray
Space Telescope. The LAT has observed the Earth during its commissioning phase
and with a dedicated Earth-limb following observation in September 2008. These
measurements yielded 6.4 x 10^6 photons with energies >100MeV and ~250hours
total livetime for the highest quality data selection. This allows the study of
the spatial and spectral distributions of these photons with unprecedented
detail. The spectrum of the emission - often referred to as Earth albedo
gamma-ray emission - has a power-law shape up to 500 GeV with spectral index
Gamma = 2.79+-0.06.Comment: Accepted for publication in PR
Fermi-LAT Study of Gamma-ray Emission in the Direction of Supernova Remnant W49B
We present an analysis of the gamma-ray data obtained with the Large Area
Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the direction of
SNR W49B (G43.3-0.2). A bright unresolved gamma-ray source detected at a
significance of 38 sigma is found to coincide with SNR W49B. The energy
spectrum in the 0.2-200 GeV range gradually steepens toward high energies. The
luminosity is estimated to be 1.5x10^{36} (D/8 kpc)^2 erg s^-1 in this energy
range. There is no indication that the gamma-ray emission comes from a pulsar.
Assuming that the SNR shell is the site of gamma-ray production, the observed
spectrum can be explained either by the decay of neutral pi mesons produced
through the proton-proton collisions or by electron bremsstrahlung. The
calculated energy density of relativistic particles responsible for the LAT
flux is estimated to be remarkably large, U_{e,p}>10^4 eV cm^-3, for either
gamma-ray production mechanism.Comment: 9 pages, 10 figure
Fermi-LAT observations of the exceptional gamma-ray outbursts of 3C 273 in September 2009
We present the light curves and spectral data of two exceptionally luminous
gamma-ray outburts observed by the Large Area Telescope (LAT) experiment on
board Fermi Gamma-ray Space Telescope from 3C 273 in September 2009. During
these flares, having a duration of a few days, the source reached its highest
gamma-ray flux ever measured. This allowed us to study in some details their
spectral and temporal structures. The rise and decay are asymmetric on
timescales of 6 hours, and the spectral index was significantly harder during
the flares than during the preceding 11 months. We also found that short, very
intense flares put out the same time-integrated energy as long, less intense
flares like that observed in August 2009.Comment: Corresponding authors: E. Massaro, [email protected]; G.
Tosti, [email protected]. 15 pages, 4 figures, published in The
Astrophysical Journal Letters, Volume 714, Issue 1, pp. L73-L78 (2010
Fermi Large Area Telescope Observations of Misaligned AGN
Analysis is presented on 15 months of data taken with the Large Area
Telescope (LAT) on the Fermi Gamma-ray Space Telescope for 11 non-blazar AGNs,
including 7 FRI radio galaxies and 4 FRII radio sources consisting of 2 FRII
radio galaxies and 2 steep spectrum radio quasars. The broad line FRI radio
galaxy 3C 120 is reported here as a gamma-ray source for the first time. The
analysis is based on directional associations of LAT sources with radio sources
in the 3CR, 3CRR and MS4 (collectively referred to as 3C-MS) catalogs. Seven of
the eleven LAT sources associated with 3C-MS radio sources have spectral
indices larger than 2.3 and, except for the FRI radio galaxy NGC 1275 that
shows possible spectral curvature, are well described by a power law. No
evidence for time variability is found for any sources other than NGC 1275. The
gamma-ray luminosities of FRI radio galaxies are significantly smaller than
those of BL Lac objects detected by the LAT, whereas the gamma-ray luminosities
of FRII sources are quite similar to those of FSRQs, which could reflect
different beaming factors for the gamma-ray emission. A core dominance study of
the 3CRR sample indicate that sources closer to the jet axis are preferentially
detected with the Fermi-LAT, insofar as the gamma-ray--detected misaligned AGNs
have larger core dominance at a given average radio flux. The results are
discussed in view of the AGN unification scenario.Comment: 28 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
We present the results of our analysis of cosmic-ray electrons using about 8
million electron candidates detected in the first 12 months on-orbit by the
Fermi Large Area Telescope. This work extends our previously-published
cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of
approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and
its validation using beam-test and on-orbit data. In addition, we describe the
spectrum measured via a subset of events selected for the best energy
resolution as a cross-check on the measurement using the full event sample. Our
electron spectrum can be described with a power law with no prominent spectral features within systematic uncertainties.
Within the limits of our uncertainties, we can accommodate a slight spectral
hardening at around 100 GeV and a slight softening above 500 GeV.Comment: 20 pages, 23 figures, 2 tables, published in Physical Review D 82,
092004 (2010) - contact authors: C. Sgro', A. Moisee
Fermi Gamma-ray Space Telescope Observations of Recent Gamma-ray Outbursts from 3C 454.3
The flat spectrum radio quasar 3C~454.3 underwent an extraordinary outburst
in December 2009 when it became the brightest gamma-ray source in the sky for
over one week. Its daily flux measured with the Fermi Large Area Telescope at
photon energies E>100 MeV reached F = 22+/-1 x 10^-6 ph cm^-2 s^-1,
representing the highest daily flux of any blazar ever recorded in high-energy
gamma-rays. It again became the brightest source in the sky in 2010 April,
triggering a pointed-mode observation by Fermi. The correlated gamma-ray
temporal and spectral properties during these exceptional events are presented
and discussed. The main results show flux variability over time scales less
than 3 h and very mild spectral variability with an indication of gradual
hardening preceding major flares. No consistent loop pattern emerged in the
gamma-ray spectral index vs flux plane. A minimum Doppler factor of ~ 15 is
derived, and the maximum energy of a photon from 3C 454.3 is ~ 20 GeV. The
spectral break at a few GeV is inconsistent with Klein-Nishina softening from
power-law electrons scattering Ly_alpha line radiation, and a break in the
underlying electron spectrum in blazar leptonic models is implied.Comment: submitted to the Astrophysical Journa
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